Rosetta team picks best landing site on surface of speeding comet

Ladies and gentlemen, we have a winner!

Early Monday morning the European Space Agency announced it had selected Site J on Comet 67P/Churyumov-Gerasimenko as the target landing spot for the Rosetta mission’s Philae lander.

The selection of the site brings the Rosetta team one step closer to being the first to land a spacecraft directly on the surface of a speeding comet.

The site is not ideal, according to the team of scientists and engineers on the ESA’s Landing Site Selection Group. However, after a meeting at CNES (the French space agency) in Toulouse, France, over the weekend, they decided unanimously that Site J is their best option.

“I’m just gratified that we found a site that everyone can agree on that is safe,” said the Jet Propulsion Laboratory’s Claudia Alexander, Project Manager and Project Scientist for NASA’s supporting role in the mission. “The most important thing is to find somewhere safe.”

Comet 67P/Churyumov-Gerasimenko is a rubber-duck shaped body about 2.5 miles across at its longest point. Rosetta spent the last 10 years looping around the solar system before it finally met up with the comet on Aug. 6.

Rosetta will continue to tag along with the comet over the next year as it zips closer to the sun, but to help scientists get a close look at the comet’s icy nucleus, Rosetta will send a small lander down to the comet’s surface.

Since August the spacecraft has circled closer and closer to the comet, eventually coming within 19 miles of it. At that distance, it can see that Comet 67P is a topographically dramatic world, with steep slopes and deep crevices and a scattering of boulders on its dark surface. That makes for pretty pictures, but is not great for finding landing sites. Part of the appeal of Site J, therefore, lies in its relatively shallow slopes, that should reduce Philae’s chance of toppling over as it lands on the surface.

There are other benefits to the site as well, scientists say. Site J appears to be mostly boulder-free, and it gets enough daily light to allow the lander to recharge its batteries. It will also provide a relatively straightforward trajectory for landing. The Landing Site Selection Group expects it will take the Philae lander about seven hours to descend to the site.

“‘J’ isn’t perfect, but overall it just rates better than the other candidate sites,” said Joel Parker, director at the Southwest Research Institute and a deputy principal investigator Alice UV spectrograph on the Rosetta mission.

The lander itself is just 30 inches high and 3 feet across, but it is equipped with 10 instruments to help scientists better understand the composition of a comet’s nucleus, and how it changes as it nears the sun.

“We don’t know if comets are ephemeral things held together like snowflakes or really had like a rock or a snowball,” Alexander said. “You can only tell by landing on it and drilling into it.”

To help Philae stick to the comet, which has very little gravity, the lander will drill into the surface with ice screws almost as soon as it lands. It also has a harpoon system that will shoot into the ground.

Philae is currently scheduled to land on Comet 67P on Nov. 11. Between now and then, the Landing Site Selection Group will be busy analyzing the site for boulder distribution and to make the final call.

“We are really nervous,” said Alexander. “There is a lot of risk, which means this may not work. People don’t appreciate that as much as they should.”

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